Typical consumers for which the invention is relevant include diesel engines for marine propulsion systems or smaller, electricity-producing power plants which selectively are operated with gas, namely such that with every working cycle a certain amount of gas under high pressure is introduced into the cylinder in addition to the diesel oil, the so-called pilot oil. The gas demand of these engines possibly changes quickly. The requested pressure is dependent on the respective power of the engine and for LNG typically lies between 150 and 300 bar. The changes in terms of pressure and mass flow are particularly pronounced and abrupt when in a group of consumers, e.g. in a power plant group, one consumer or engine suddenly fails completely, e.g. due to an emergency shut-off. The apparatus which supplies such consumers with gas must be able to comply with such changes.
An approach for the solution of this problem consists in that a large amount of evaporated, i.e. gaseous gas under high pressure oriented towards the maximum pressure of the consumer constantly is provided, in order to be able to compensate the fluctuations in consumption.
However, this procedure is very expensive in terms of safety, because a larger amount of gas which is under high pressure is dangerous on principle.
Another approach consists in that the gas is pressurized by means of a pump when it still is in the liquid state, an excess thereof, based on the actually required mass flow, is withdrawn from the storage tank, and the partial quantity not required again is recirculated into the storage tank. Since the liquid gas also heats up due to the increase in pressure, heat constantly is introduced into the store of cryogenic liquefied gas with the consequence that the undesired formation of boil-off gas in the storage tank is increased. Due to the related increase in pressure in the storage tank, which is not designed for high pressure, this represents a problematic source of danger in particular on board a ship.
If one renounces the provision of buffer quantities of gas, which is under pressure, and attempts to satisfy the requirements of the consumer in terms of pressure and mass flow of the supplied gas alone with the usual means of control, one quickly is faced with limits. The control either is too slow to be able to reproduce a short-term pressure increase or pressure drop taking place within few seconds, or it tends to uncontrolled feedback oscillations of the gas pressure.